1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image onto paper.
2. Description of the Related Art
In an image forming apparatus such as a printer, a copier, or a multi-function device, paper supplied from a paper containing portion is conveyed. The image forming apparatus performs printing, at a predetermined position, onto the paper which is being conveyed. The image forming apparatus has an internal paper path on which rollers are provided at intervals each of which is shorter than the length of paper in the longitudinal direction. The image forming apparatus controls the rotary drive of the rollers to allow the paper to pass through the positions on the paper path at an appropriate time.
In order to start/stop the rotary drive of the rollers, a clutch is often used. The clutch is provided between the rollers and a motor operating as the drive source of the rollers to connect/disconnect transmission of the rotational driving force. This enables the rollers to stop with the motor remains rotating. The clutch is used, for example, for the case where one motor is used in common as the drive source of rollers which rotate at different times.
The clutch has a delay in its response to control signals. To be specific, one example of the delay is an engagement delay in response to a command to switch from a disengagement state where the rollers and the motor are disengaged from one another to an engagement state where the rollers and the motor are engaged with one another. Another example of the delay is a disengagement delay in response to a command to switch from the engagement state to the disengagement state. The engagement delay and the disengagement delay vary according to the individual differences of clutches. Further, a difference in thickness of paper makes a difference in torque applied to the rollers (load from the motor side), leading to variations in the engagement delay and the disengagement delay.
In relation to control of a clutch in an image forming apparatus, a sheet conveyance device has been proposed. In the sheet conveyance device, data on stop characteristics of rollers with clutches disengaged are stored, and engaging timing of the clutches is controlled based on the data stored (Japanese Unexamined Patent Application Publication No. 2002-370845).
Another technology has been proposed in which an engagement time (engagement delay) from a time point at which clutches are engaged to a time point at which variations in rotational speed of the motor falls within a predetermined range is measured, and a time at which an engagement command is given to the clutches after the clutches are disengaged is determined based on the engagement time (Japanese Unexamined Patent Application Publication No. 2012-140212).
In a print job of conveying sheets of paper continuously, it is desirable to minimize a gap between sheets of paper (inter-sheet space) to improve the productivity of printing.
According to the technology disclosed in Japanese Unexamined Patent Application Publication No. 2002-370845, the sheet conveyance is controlled based on the data, stored in advance, related to response delay. Therefore, it is difficult to deal with variations in response delay due to mechanical differences in image forming apparatuses, variations in load, change in environment, aging, and so on. Stated differently, since the response delay is not actually measured, it is necessary to expect variations in a certain length of response delay. It is also necessary to control the clutches at a time when margins are provided to avoid having an excessively small inter-sheet space even if the response delay is largest within the expected range of delay. Unfortunately, this makes it impossible to minimize the inter-sheet space.
In contrast, according to the technology described in Japanese Unexamined Patent Application Publication No. 2012-140212, the response delay is actually measured based on the rotational speed of the motor. Therefore, minimizing an inter-sheet space is possible in principle.
A technique for detecting a change in rotational speed of a motor is applicable only to a motor having rotational speed varying in accordance with torque, e.g., a brushless DC Motor. Unfortunately, the technique is not applicable to other types of motors. Therefore, in using a synchronous motor having no variations in rotational speed, e.g., a stepper motor, for paper conveyance, minimizing an inter-sheet space is unfortunately impossible.